TypeError: unhashable type: 'list'

You used a list where Python needs a value that can never change: in a set, as a dictionary key, or in a set or dict lookup. Use a tuple instead.

PythonType errors

What it means

Sets and dictionaries find items by their hash, and a hash is only valid while the value stays the same. Lists can be changed, so Python refuses to hash them. Newer versions add context, such as "cannot use 'list' as a set element". Tuples, strings and numbers are hashable; lists, dicts and sets are not.

Common causes

1. Adding a list to a set

Common when collecting coordinates or pairs.

Breaks

seen = set()
seen.add([x, y])

Works

seen = set()
seen.add((x, y))

2. Using a list as a dictionary key

Keys must be hashable for the same reason.

Breaks

visits = {}
visits[[lat, lng]] = 1

Works

visits = {}
visits[(lat, lng)] = 1

3. Removing duplicates from a list of lists

set() has to hash every element, and the elements are lists.

Breaks

unique = set(pairs)

Works

unique = {tuple(p) for p in pairs}

How to find it in your own code

The failing line is the one that hashes: set(), .add(), a dictionary key, or `in` on a set or dict. Convert the value with tuple(value) before storing it, and back with list() only if you need to change it later.

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Other common errors

TypeError: '>' not supported between instances of 'str' and 'int'TypeError: list indices must be integers or slices, not float TypeError: can only concatenate str (not "int") to str ValueError: invalid literal for int() with base 10 AttributeError: 'str' object has no attribute TypeError: object of type 'int' has no len()TypeError: sequence item 0: expected str instance, int found TypeError: unsupported operand type(s) for +: 'int' and 'str'